In recent years, there has been an intense interest in receiving signals in the millimeter-wave region of the electromagnetic spectrum, roughly designated as the frequency band from 18 GHz to 100 GHz. There is a need for a receiver that can be readily coupled to an antenna to minimize losses in the interconnection between the antenna and the receiver, in order to obtain maximum sensitivity. The receiver and antenna combination is often used as one channel of an n-channel direction finding (DF) system in which a plurality of the receiver/antennas are disposed at different locations around a host platform, such as an aircraft, for example, so that it is possible to form a monopulse ratio between the signals received at two adjacent antennas to thus provide DF information. Such techniques are known to those skilled in the art, and are described in detail in my book, Microwave Passive Direction Finding, published by John Wiley & Sons, New York, 1987.
Antenna sensor systems that are required to provide monopulse DF are dispersed around the host platform essentially to envelop it in order to eliminate ground effects, and to obtain an optimum field of view. In the past, dispersed antenna and crystal video detector receivers have provided amplitude or phase monopulse DF data by comparing the amplitude or phase of signals from two or more adjacent antennas which are physically separated. The most common configuration is a four antenna system using spiral antennas equally squinted or pointed in each of four quadrants. A video detector and logarithmic video amplifier recovers the amplitudes of the strongest adjacent signals, and forms the monopulse ratio of the two by video logarithmic subtraction, thus yielding a line-of-beating of the intercept. Phase monopulse methods utilize the path length or time-difference-of-arrival of a signal, as intercepted by two or more antennas, as a phase angle from which the same DF data may be obtained.
One type of antenna receiver device suited to DF applications is described in my U.S. Pat. No. 4,573,212, assigned to the same assignee as the present application. In the device described in my earlier patent, a mixer diode is connected across the two output terminals of a two-element spiral antenna. An oscillator signal and dc bias are applied to the mixer diode by direct and capacitive coupling. The non-linear properties of the mixer diode multiply the oscillator signal and an incoming RF signal received by the antenna, resulting in an intermediate frequency signal which is the product of the oscillator signal and the received RF signal. The intermediate frequency signal is recovered at the outer extremities of the spiral antenna for signal processing.
While effective for DF and other applications, the antenna receiver device disclosed in U.S. Pat. No. 4,573,212 does have certain limitations. As with slow-wave antennas in general, the spiral antenna disclosed in U.S. Pat. 4,573,212 will exhibit either odd or even mode behavior depending on whether the signal currents at the feed terminals of the spiral elements are in anti-phase or in-phase relationship, respectively. Each mode exhibits different antenna radiation characteristics. In the first or odd mode, the radiation is perpendicular to the plane of the spiral and has a single peak along the principal or boresight axis. In the normal or even mode, however, there is a null along the principal or boresight axis. For this reason, it is usually preferred to use the first type of antenna/receiver device in the odd mode, which requires that a balun or other coupling, which attains or excites the anti-phase relationship of the signals at the antenna input terminals, be used to couple the antenna output terminals to detecting and processing circuitry. Use of a balun in the antenna described in U.S. Pat. No. 4,573,212 would increase its complexity, especially at millimeter-wave frequencies, and could contribute to signal attenuation and degradation of the performance of the device.
The device disclosed in U.S. Pat. No. 4,573,212 makes use of an oscillator to provide the signal to mix with the inputs from the antennas. In more recent technology, this oscillator is usually included as part of each of the individual antenna/receiver assemblies, which are dispersed about the aircraft. As a result, each of these assemblies may encounter extremes of vibration, shock and temperature, which may be different at each location, causing an error in the DF measurement since amplitudes or phases are compared for DF determination, as described above.
It is also often essential to measure the frequency of an input signal, which can be done by knowing the local oscillator frequency and measuring the downconverted signal at the intermediate frequency that is coupled from the outer extremities of the antenna. The aforementioned environmental conditions can cause sufficient frequency modulation of an unstabilized self-contained oscillator, in the dispersed antenna/receiver units, to render this frequency measurement useless. To overcome this difficulty, injection or phase locking of the self-contained local oscillator is required. This may be accomplished by transporting a stable reference or pilot signal from a benign location to each of the "n" antennas/receivers for this purpose. Since the stable signal must be conveyed some distance, it often undergoes attenuation when carried by standard coaxial cable, and may pick up undesirable interference. This renders the device susceptible to electromagnetic interference and jamming. This is because interfering signals lie in the same frequency range as the local oscillator, received signals, and IF signals, and despite the use of metallic semi-rigid conductor coaxial cables and other methods of conveyance of these frequencies, the IF signals may be interfered with, due to coupling and pickup effects.
It is often advantageous, also, to recover video information contained in modulated RF signals. Instead of mixing the RF signals to obtain a downconverted IF signal, as in U.S. Pat. No. 4,573,212, the RF signal may be applied to a detector whose output is a video signal representative of the video information.
It is an object of the present invention to provide an antenna/receiver which overcomes the drawbacks of prior antenna/receivers mentioned above, and which does so in an elegant, inexpensive and easy-to-implement fashion.
It is a further object of the present invention to provide a novel and improved apparatus and method using fiber-optic technology to accomplish the objective of distributing signals with minimum loss and interference susceptibility.